Random access is a fundamental component of every cellular communications network. In general, random access enables a wireless device, which in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standards is referred to as a User Equipment (UE), to request a connection setup. Random access may be used for various purposes including establishing a radio link when initially accessing the cellular communications network, re-establishing a radio link after radio link failure, establishing uplink synchronization for a new cell for handover, etc. As illustrated in FIG. 1, in 3GPP LTE, the random access procedure is performed after first performing a cell search procedure. More specifically, an evolved Node B (eNB) 10 broadcasts Primary and Secondary Synchronization Signals (PSS/SSS) and system information (step 1000). A UE 12 performs a cell search procedure whereby the UE 12 synchronizes to the downlink timing of the cell served by the eNB 10 by detecting the PSS/SSS (step 1002). The UE 12 then obtains, or reads, the system information (step 1004). The system information includes various types of information including information that identifies physical time and frequency resources to be used by the UE 12 for random access.
With respect to the random access procedure, the UE 12 transmits a random access preamble (step 1006). The random access preamble is transmitted on a Random Access Channel (RACH), which is a logical transport channel. The RACH is mapped into a Physical RACH (PRACH), which is provided on time and frequency radio resources indicated by the system information broadcast by the eNB 10. The eNB 10 detects the random access preamble transmitted by the UE 12 and, based on a random access sequence transmitted therein, determines the uplink timing for the UE 12 (step 1008). The eNB 10 then transmits a random access response to the UE 12 including a timing adjustment for the uplink from the UE 12 (step 1010). The UE 12 adjusts its uplink timing according to the timing adjustment received in the random access response (step 1012). The UE 12 and the eNB 10 then use Radio Resource Control (RRC) signaling to exchange information to complete establishment of the radio link between the eNB 10 and the UE 12 (steps 1014 and 1016).
As illustrated in FIG. 2, the random access preamble, which is also referred herein to as a RACH preamble, includes a sequence (referred to herein as a RACH sequence) having a time duration of TSEQ, and a Cyclic Prefix (CP) having a time duration of TCP. The CP is added to the RACH sequence in order to reduce Inter-Symbol Interference (ISI). The RACH sequence is a NZC-point Zadoff-Chu (ZC) sequence, wherein NZC=839. NZC is the length of the ZC sequence and thus the length of the RACH sequence. In 3GPP LTE, cell sizes up to approximately 150 kilometers (km) (radius) are supported. In order to provide this support, the time duration of the RACH sequence (TSEQ) must be significantly greater than the round-trip time for the largest supported cell size. Specifically, 3GPP LTE defines four random access configurations (Configurations 0-3). For each configuration, the RACH sequence spans one or more 0.8 millisecond (ms) (transmission) cycles. The typical random access configuration is Configuration 0. In Configuration 0, the RACH sequence is a 0.8 ms sequence and, as such, the RACH sequence spans only one 0.8 ms cycle. In particular, in Configuration 0, TSEQ=0.8 ms, TCP=0.1 ms, and a guard time (not shown) is also equal to 0.1 ms. Configuration 0 allows for cell sizes (radius) of up to 15 km. In order to support even larger cell sizes (i.e., up to 150 km), Configurations 1-3 use longer CPs and, in the case of Configurations 2 and 3, longer sequence lengths (i.e., TSEQ=1.6 ms), but over multiple subframes. For example, in Configuration 2, TSEQ=1.6 ms, TCP=0.2 ms, and the guard time (not shown) is also 0.2 ms. In Configuration 2, the RACH sequence (duration of TSEQ=1.6 ms) spans two 0.8 ms cycles. However, each cycle has a duration of 0.8 ms, which corresponds to a subcarrier frequency spacing (ΔfPRACH) for the PRACH subcarriers of 1.25 kilohertz (kHz) (i.e., ΔfPRACH=1/TCYC=1/0.8 ms=1.25 kHz, where TCYC is referred to herein as the cycle time).
The PRACH used to transmit the RACH preamble is 6 Resource Blocks (RBs) in the frequency domain. In the time domain, the PRACH is either 1 subframe (1 ms) (Configuration 0), 2 subframes (2 ms) (Configurations 1 or 2), or 3 subframes (3 ms) (Configuration 3). FIG. 3 illustrates the PRACH for Configuration 0. As illustrated, in order to fit the 0.8 ms sequence into 6 RBs in the frequency domain and provide orthogonally between the PRACH subcarriers, the subcarrier frequency spacing (ΔfPRACH) for the PRACH subcarriers is 1.25 kilohertz (kHz) (i.e., ΔfPRACH=1/TCYC=1/0.8 ms=1.25 kHz). Thus, as illustrated, the subcarrier frequency spacing (ΔfPRACH) for the PRACH subcarriers is 1/12th of the subcarrier frequency spacing (ΔfTRAFFIC) for the subcarriers of the other uplink channels (e.g., Physical Uplink Shared Channel (PUSCH)), which is 15 kHz. There are 864 PRACH subcarriers within the 6 RBs allocated for PRACH. Of these 864 PRACH subcarriers, 839 PRACH subcarriers are used for transmissions of an 839-point ZC sequence.
One issue with the conventional PRACH of 3GPP LTE is that, due to the large number of PRACH subcarriers, processing of the PRACH at both the transmitter and the receiver is complex. In particular, a conventional RACH preamble transmitter 14 is illustrated in FIG. 4. As illustrated, a RACH sequence for the RACH preamble (in the time domain) is input to a Discrete Fourier Transform (DFT) (e.g., Fast Fourier Transform (FFT)) function 16 that performs an NZC-point FFT. Again, for 3GPP LTE, NZC=839. The RACH sequence is an 839-point ZC sequence. The cycle time, or duration, (TCYC) of the RACH sequence is 0.8 ms and, as such, the frequency spacing of the frequency bins at the output of the FFT function 16 is 1/TCYC=1.25 kHz. A subcarrier mapping function 18 maps the outputs of the FFT function 16 to the appropriate PRACH subcarriers within the uplink system bandwidth.
The outputs of the subcarrier mapping function 18 are provided to corresponding inputs of an Inverse Discrete Fourier Transform (IDFT) (e.g., Inverse FFT (IFFT)) function 20. The size of the IFFT 20 (referred to here as NDFT) is TCYC·fs, where fs is the sampling rate. For a 20 Megahertz (MHz) system bandwidth, 3GPP LTE uses a sampling rate of 30.72 MHz and, as such, the size of the IFFT 20 is 24,576 (i.e., NDFT=TCYC·fs=800 microseconds (μs)·30.72 MHz). The large size of the IFFT 20 leads to a significant amount of resources and complexity when implementing the RACH preamble transmitter 14. A repeat function 22 repeats time domain sequence output by the IFFT 20, if needed, according to the random access configuration. Lastly, a CP insertion function 24 inserts the CP to thereby output the final time domain RACH preamble for transmission.
In the same manner, small RACH subcarrier spacing results in complexity at the conventional RACH preamble receiver. As illustrated in FIG. 5, a conventional apparatus 26 includes a normal traffic path 28 and a RACH path 30, where the RACH path 30 is a conventional RACH preamble receiver. The normal traffic path 28 includes a data processing portion 32, which includes a CP removal function 34, a frequency shift function 36, and a symbol FFT function 38. The CP removal function 34 removes the CP of a receive signal. The frequency shift function 36 then shifts the frequency of the received signal by ½ of the normal subcarrier spacing (i.e., ½·15 kHz=7.5 kHz). The received signal is then divided into time pieces corresponding to a fraction (e.g., 1/14 or 1/12) of a millisecond, where these pieces are referred to as symbols. The symbol FFT function 38 then performs an FFT per symbol. In particular, for a 20 MHz bandwidth, the symbol FFT function 38 performs a 2,048 point FFT per symbol. The resulting frequency domain signal pieces are then provided to an uplink processing function 40 for further signal processing.
For the RACH path 30, a “super FFT” function 42 performs an FFT for 0.8 ms of samples of the received signal. For a 20 MHz bandwidth, the size of the FFT is 24,576. Thus, due to the large size of the FFT, the FFT is referred to herein as a “super FFT.” The super FFT function 42 involves a large amount of data to transport and buffer and requires a large amount of computation. The output of the super FFT function 42 is then provided to a data processing portion 44. The data processing portion 44 includes a RACH subcarrier selection function 46, a correlation function 48, and an IFFT function 50. The RACH subcarrier selection function 46 selects the 839 outputs of the super FFT function 42 that correspond to the RACH subcarriers. The correlation function 48 then correlates the output of the RACH subcarrier selection function 46 with known ZC sequences to thereby extract a temporary identifier of the transmitting UE. More specifically, the correlation function 48 performs a multiplication of the received RACH subcarriers with the conjugate of one of the known ZC sequences in the frequency domain. This effectively simultaneously does the correlation at all time shifts of that ZC sequence in one step. The IFFT function 50 then performs a 2,048 point IFFT resulting in a time domain signal that is then processed by a RACH detection module 52. The output of the IFFT function 50 shows where in time any correlation peaks are located. Notably, the correlation (in the frequency domain) and the IFFT are performed once for each of the desired ZC sequences. The super FFT function 42 is a substantial burden in terms of storage space and power, while most of the outputs of the super FFT function 42 are discarded in the data processing portion 44.
U.S. Pat. No. 8,634,288 B2, entitled SYMBOL FFT RACH PROCESSING METHODS AND DEVICES, which was filed on Jun. 1, 2011 and issued on Jan. 21, 2014, describes systems and methods for extracting a RACH preamble without using a super FFT. One embodiment of an apparatus 54 as disclosed in U.S. Pat. No. 8,634,288 B2 is illustrated in FIG. 6. As illustrated, the apparatus 54 includes a device 56 for extracting the RACH preamble from a received signal in a manner that eliminates the need for a super FFT. In particular, the apparatus 54 includes a traffic path and a RACH path. The traffic path is the same as that of FIG. 4. However, the RACH path includes the data processing portion 32 (which is also used for the traffic path), the device 56, the data processing portion 44 for the RACH path, and the RACH detection module 52. Unlike the conventional apparatus 26 of FIG. 4, the apparatus 54 of FIG. 5 uses the data processing portion 32 of the traffic path as part of the RACH path along with the device 56 to eliminate the super FFT function 42. As a result, complexity is substantially reduced.
In particular, for the RACH path, the output of the symbol FFT function 38 for a predetermined number of symbols (e.g., 12) is input into the device 56, one by one. Within the device 56, a de-mapping function 58 selects a portion of the signal where the RACH should be located at that point in time. Due to the coarser FFT (i.e., the FFT performed by the symbol FFT function 38), the selected portion of the signal, which spans about 1 MHz, covers about 72 distinct frequencies in the output spectrum of the symbol FFT. The selected portion of the signal (where all other non-RACH frequency bins have been set to zero) is shifted to baseband.
An IFFT function 60 performs a 256-point IFFT on the selected portion of the signal to thereby transform the selected portion of the signal back to the time domain. A phase adjustment function 62 performs a phase adjustment to compensate for the group delay of the symbol CP gaps when moving the data to baseband (the phase of the first sample of the IFFT output may be zero or another value, which is not necessarily equal to the phase of the signal at the end of the CP time). A CP zero insertion function 64 inserts zeroes into the symbol CP times, and a downsampling function 66 downsamples the signal by a factor of 3. The downsampling occurs to limit the number of points in a sequence corresponding to the RACH preamble to a number of points necessary and relevant (the number of 256 points used in the IFFT function 60 being in excess of 3·72, which is the number of frequencies corresponding to the RACH band after de-mapping, and this number being further increased by the symbol CP insertion). The data processing at functions 58-66 is performed for each of the symbols considered (e.g., the number of symbols may be 12).
The output of the downsampling function 66 is accumulated by an accumulation function 68, and the RACH preamble portion is then selected by a preamble selection function 70. An FFT function 72 then performs a 1,024-point FFT. The frequency spacing of the output bins of the FFT function 72 is 1.25 kHz, and 839 of the output bins of the FFT function 72 correspond to the 839 PRACH subcarriers. The output of the FFT function 72 is then input to the data processing portion 44, where processing proceeds in the manner discussed above. Thus, by using the device 56, the output of the symbol FFT function 38 can be used for RACH extraction. However, since the frequency spacing of the outputs of the symbol FFT function 38 is 15 kHz, the device 56 operates to recover the PRACH subcarriers, which have a 1.25 kHz subcarrier spacing, from the outputs of the symbol FFT function 38, which have a 15 kHz spacing.
The systems and methods of U.S. Pat. No. 8,634,288 B2 provide substantial benefits in terms of reduced complexity. However, both in the conventional RACH receiver used in the apparatus 26 of FIG. 5 and the RACH receiver implemented in the apparatus 54 of FIG. 6, normal traffic (e.g., PUSCH traffic) results in interference during RACH detection and vice versa. As such, there is a need for systems and methods that reduce or eliminate interference between RACH transmissions and normal traffic transmissions.